At present, in the field of wireless network, with the rapid development of wireless local area network (WLAN), the application range of the WLAN are being widen day after day. In order to cope with various network requirements, the specification group of the Institute of Electrical and Electronic Engineers IEEE802.11 has issued a series of most ordinary WLAN technical standards, such as 802.11ah, 802.11b, 802.11g and 802.11n. Other task groups which are devoted to the development of improved standards of the existing 802.11 technology are established in succession subsequently. For example, with the development of the Internet of Things, the IEEE has established a 802.11ah task group, the primary task of which is to modify and enhance the medium access control (MAC) and physical (PHY) layer of the WLAN, so as to adapt to the requirements of networks such as a Smart Grid, a sensor network, an Environmental/Agricultural Monitoring and an Industrial Process Automation network.
In a wireless local area network, an access point (AP) and a plurality of non-AP stations (STAs) associated with this AP compose a Basic Service Set (BSS). Before using the service of the BSS, the STA needs to accomplish an authentication and association process with the AP. If the association succeeds, the AP allocates an association identifier (AID) to the STA, wherein the AID is the identity of the STA in the current BSS, i.e. this STA is able to be distinguished from other STAs via the AID; while STAs belonging to different BSSs may use the same AID. At the same time, when a plurality of BSS are connected via a Distribution System (DS), they can form an Extended Service Set (ESS). A plurality of STAs can also form a self-organizing wireless local network, which is called an independent BSS (IBSS), and the STAs in the IBSS can communicate directly with each other.
In the new WLAN application scenario defined by the 802.11ah task group, some typical examples require the amount of the supported user as large as possible. For example, during intelligent metre reading, it is required that at most 6000 stations can be supported under one BSS, each station is distributed with an AID during association, and the system require data of these stations to be reported periodically or non-periodically. The stations subordinated to these applications are powered substantially by a battery, and it is typically required that there is no need to change the battery in a long period of time. Therefore, these stations may enter the power saving mode when not performing any transceiving operation. For a station in a power saving mode, the AP notifies the station whether there is buffer data to be sent, by sending paging information, i.e. a Traffic Indication Map (TIM) information element.
FIG. 1 is a schematic diagram of a traffic indication map information element according to the relevant art. As shown in FIG. 1, the value of the length field of the TIM information element is the sum of a constant length (2 octets), the length of a bitmap control field and the length of the virtual bitmap (Partial Virtual Bitmap). The partial virtual bitmap in the TIM information element is the portion in the Traffic-Indication Virtual Bitmap (abbreviated as bitmap below) which indicates that there is station traffic, the octet number of the starting position of the partial virtual bitmap in the bitmap is N1, and the octet number of the end position thereof is N2. The bitmap has 251 octets in total, which is 2008 bits. Each bit corresponds to one AID, and when it is set to 1, it indicates that the buffer data of the station with this AID is to be sent. 7 octets in the bitmap control field are used for indicating a bitmap offset, and the value of the bitmap offset is set to N1/2. In the relevant art, N1 may be the maximum even number satisfying that the bit numbers 1˜((N1*8)−1) in the bitmap are all 0, and N2 may be the minimum number satisfying that the bit numbers ((N2+1)*8)˜2007 in the bitmap are all 0.
The length of the length field of the TIM information element is 1 octet, and each bit therein can further indicate a length of 1 octet, and the range that can be indicated thereby is 0˜(28−1) words. In addition, the bitmap offset is 7 bits, which can indicate an offset of an even number, and can at most indicate an offset of 254 octets. Limited to the value of the length field of the TIM information element, the partial virtual bitmap can only indicate whether stations corresponding to 2007 AIDs at most have buffer data to be sent, which is clearly not able to satisfy the requirement of supporting a large number of users.
Aiming at the above-mentioned problem, in the relevant art, the bitmap is statically segmented or dynamically segmented to support a large amount of users. In one beacon interval, the number of stations for which the AP can serve is limited. For the case where the number of users is large, the bitmap is compressed in segments, and theses compressed segments are sent in a plurality of beacon intervals respectively. A plurality of methods for compressing a TIM in segments are given in the relevant art. The TIM compressed in segments can be abbreviated as a compressed TIM, wherein the methods for compressing a TIM mainly include Hierarchical TIM compression and Offset+Bitmap compression. FIG. 2 is a schematic diagram of hierarchical TIM compression according to the relevant art. As shown in FIG. 2, hierarchical TIM compression mainly divides the TIM into a plurality of pages, divides each page into a plurality of blocks, and divides each block into a plurality of sub-blocks, thus carrying in the compressed TIM only non-zero sub-blocks of the bitmap, beneficial for reducing the amount of non-zero sub-blocks of the bitmap in the compressed TIM, wherein TBD (To Be Discussed) indicates that the settings of the field are undetermined. FIG. 3 is a schematic diagram of offset+bitmap compression according to the relevant art. As shown in FIG. 3, the offset+bitmap compression mainly indicates, by both segment offset and the position in the segment bitmap in the TIM of different segments, the AIDs of the stations the buffer data of which is to be sent by the current AP, thus supporting a large number of users, wherein an offset size field dynamically indicates the size of the offset field in a segment.
However, how the AP indicates a plurality of non-AP stations (STAs) in the case where there is not unicast buffer data of any station is not considered in the relevant art.